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Post subject: Bassman AB165/AA864 Tone circuit troubleshooting...
Posted: Tue Mar 15, 2011 9:36 am
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So... I just got done doing the Andy Ruhl Mod to my 67 SF Bassman. While I was doing the mods I also replaced all of the caps, tubes, resistors, diodes, bias pot, normal channel vol pot, slide switches, and power tube sockets.

I get everything back together and with my fingers crossed I flip the power switch... IT WORKS!!! (joy) I then proceed to set the bias and I plug my guitar in to make sure she really works... I flipped the stand-by switch and notice some popping at first (hmm...) and then started to mess withe the tone knobs and noticed they were not doing what they should be doing. I flipped the "bright" switch and I had no effect what so ever. The Treble knob is acting like a gain/vol and the bass has no effect what so ever. On the bass channel the deep switch worked but the bass pot wasn't working and the treble wasn't much different. Both volumes work fine.

I followed the mods and triple checked my work as I moved down the line but this tone circuit is throwing me for a loop :?:

below is the mod descriptions

Pre amp - Take out the 500pf and .01 caps across the 100k plate load resistors. That will open up the high end. There's nothing else to
do until you get to the phase inverter!


Power amp - Change the coupling cap to the phase inverter to a .001. Make the feedback loop look like the AA864 by using an 820
ohm resistor and connecting it to the high side of a 100 ohm resistor like a standard blackface amp has. Change the .022 caps to .1. I
actually use .047's here because I think it tightens up the bass a little bit. Don't forget to reverse the speaker side of the output
transformer by connecting the green to positive and the black to ground.


Bias supply - Make it look like any other blackface amp. I normally use a 50uf-50v filter cap, but anything higher than that up to about
100uf is fine. Use 220k grid resistors instead of the 100k's that may have come later on some AB165's..


Power supply - Change all the electrolytics with original values. The extra 16uf in the power supply of the AB165 (but labeled as an 8uf
on the schematic) is a welcome addition and should stay. It would be a good idea to change the carbon comp resistors under the cap
pan with some metal oxide or metal film ones to reduce noise and popping (or the chance of having it happen later on).



Can someone please point me in the right direction? :?


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Post subject: Re: Bassman AB165/AA864 Tone circuit troubleshooting...
Posted: Tue Mar 15, 2011 11:14 am
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I don't know Andy Ruhl and I have no idea what his "mods" are intended to accomplish, given that the AA864/AB165 Bassman is already a fine amp. But on the face of it, I'd ditch those pre-amp alterations. All of them.

JMO

Arjay

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Post subject: Re: Bassman AB165/AA864 Tone circuit troubleshooting...
Posted: Tue Mar 15, 2011 12:03 pm
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I would go with Retroverbal, i would put back all the pre-amp stuff. You just ditched and bypassed all the tone shaping of the amp. Put them back and you should be fine.




I would also like to see the instruction for this mod.


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Post subject: Re: Bassman AB165/AA864 Tone circuit troubleshooting...
Posted: Tue Mar 15, 2011 12:06 pm
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Arjay,
Are you saying put the caps back across the plate load resistors??? What about the tone circuit problems I'm having? Would those caps not being there cause those pots not to do what they are suppose to? Those caps are there to bleed treble to ground right? The amp is a AB165 that I blackfaced (Andy Ruhl mods). His mod on a 165 is a hybrid of the AB165 and AA864.
http://acruhl.freeshell.org/mga/main/AA864_vs_AB165.html

The only thing that I can think of it being is if I wired the slide switches in wrong, because I have checked the wiring to the schematic/layout and it all checks out as far as the pots go.


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Post subject: Re: Bassman AB165/AA864 Tone circuit troubleshooting...
Posted: Tue Mar 15, 2011 12:23 pm
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Here is the mod, well some of it.
Image


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Post subject: Re: Bassman AB165/AA864 Tone circuit troubleshooting...
Posted: Tue Mar 15, 2011 12:52 pm
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pinhead88 wrote:
Arjay,
Are you saying put the caps back across the plate load resistors??? What about the tone circuit problems I'm having? Would those caps not being there cause those pots not to do what they are suppose to?

The only thing that I can think of it being is if I wired the slide switches in wrong, because I have checked the wiring to the schematic/layout and it all checks out as far as the pots go.


Those caps you took out are the tone circuit they are attached to the pots.that's why they are not working and the switch is attached to the pots.So that explains why the tone controls and switch is not working so if you install them back you tone and switch will work.

As with the driver that is a standard black face mod and usually sounds better then the stock SF


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Post subject: Re: Bassman AB165/AA864 Tone circuit troubleshooting...
Posted: Tue Mar 15, 2011 12:54 pm
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Yikes! Lots of mods there!

Have to say I'd go back and make things as they were.

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"You can't spend what you ain't got, you can't lose what you ain't never had" ~ McKinley Morganfield


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Post subject: Re: Bassman AB165/AA864 Tone circuit troubleshooting...
Posted: Tue Mar 15, 2011 2:03 pm
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pinhead88 wrote:
The amp is a AB165 that I blackfaced.


Really......

:shock:

Just what do you think constitutes a "blackface AB165"?

The circuit design was finalized in January of 1965......it already is a blackface.

Arjay

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Post subject: Re: Bassman AB165/AA864 Tone circuit troubleshooting...
Posted: Tue Mar 15, 2011 2:36 pm
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Damn! Just tried to post this wonderful dissertation and the dang thing dropped the entire message into the nether region of Net-dom... sigh :mrgreen:

Couple things or three:

1.) The 100k-ohm to 220k-ohm resistor change is NOT correct. The 100K-ohm resistor of the AB165 ties onto the 12AT7 anode plate. The 220k-ohm resistor of the AA864 loads the power tube grids.

2.) Changing the PI coupling cap from stock 0.1mfd to 0.001 mfd makes no sense either. This is a whopping change in the LF "cutoff knee."

3.) Your amp (AB165) bias power supply has different layout than the AA864. Because, the AB165 adjusting pot is for hum balance. And the AA864 is for bias adjustment. I'd be very careful doing any wholesale value changes to components in this section of the amp.

4.) And caps in the pico-farad range should NOT be arbitrarily removed. They are usually placed in amps to reduce instabilites. Yes, they cut down the HF. But, this maybe because too high HF cutoff in this particualr amp, leads to parasitic instabilities and fried tubes.

5.) Good to have a tonal endpoint, so that any mods you are doing is heading in that direction. And do mods one-step-at-a-time. Test for changes. And any stability, overheating, etc issues. Then, move on to the next (but, only if you feel you need more tweaking toward your endpoint).

6.) Watch out about that global feedback change (47k-ohm to 820-ohm). The AB165 has a cap in series tied to that resistor. Kinda like a mini-crossover network. I feel a little uneasy about tossing NFB values around. Gotta be careful. Have some dB goal in mind. Aiken has a nice blurb about designing GNFB circuits. See link (click onto TECH INFO, then ADVANCED:

http://aikenamps.com/

Like others have stated, you may need to go back and re-install all of the parts you replaced. See if all is ok. Then, if you want to modify. Baby steps. If there are problems = lot easier to find them.

Good luck with your amp. Keep us posted!


Last edited by BMW2002Ti on Tue Mar 15, 2011 3:17 pm, edited 2 times in total.

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Post subject: Re: Bassman AB165/AA864 Tone circuit troubleshooting...
Posted: Tue Mar 15, 2011 2:39 pm
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BMW2002Ti wrote:
Baby steps.


+......1......!

Arjay

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Post subject: Re: Bassman AB165/AA864 Tone circuit troubleshooting...
Posted: Wed Mar 16, 2011 2:28 am
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OK.... put the caps back in and I wasn't surprised that there wasn't any change what so ever. I fixed the Bass channel by removing the resistor on the bass pot to ground and connecting it straight to ground (like on the AA864) now its fine and sounds damn good for a bass channel (with a guitar). As for the Normal channel still giving my brain a fuzzy feeling :roll: . Before, the problem was Volume was operating normal, the treble pot was not giving much "treble", and the bass pot was inop. I thought there might be a ground issue so I checked continuity to all the grounds and checked fine. I checked the wiring from before the preamp tube up to the pots and all checked out ok. On the bass pot the resistor wasn't grounding out so I bi-passed it and the bass pot works. I checked the schematic and it was the correct value that I replaced... I even replaced both tone pots with known working ones to check and see if it was a pot issue but it wasn't the issue. Now for THE BRIGHT SWITCH .... It isn't doing anything so I put the old one in and same problem. I checked continuity here and when the switch is supposedly turned on its not closing the circuit. The new switches are six pronged and check fine when out of the circuit but when I put it back in the problem occurs.

As for "blackfacing" the amp I guess I should have used the term "Pre CBS-ing" unlike other amps the bassman circuit saw their hands sooner than later.

I took the article and schematic for the AB165/AA864 to a local amp guru and we talked through the the circuit and discussed each mod that had been done and he didn't disagree with any of the mods on paper. He also said that I shouldn't have done all of it at once as well :shock: . I learned this lesson the hard way and it won't be forgotten. :oops:

below is the schematic for the hybrid
Image


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Post subject: Re: Bassman AB165/AA864 Tone circuit troubleshooting...
Posted: Wed Mar 16, 2011 6:18 am
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As for the normal channel have you checked the caps for DC leakage.that might explain the off tones you might have


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Post subject: Re: Bassman AB165/AA864 Tone circuit troubleshooting...
Posted: Wed Mar 16, 2011 9:05 am
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joncarr1979 wrote:
As for the normal channel have you checked the caps for DC leakage.that might explain the off tones you might have


Not the OP but for my own knowledge, how is this done?

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"You can't spend what you ain't got, you can't lose what you ain't never had" ~ McKinley Morganfield


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Post subject: Re: Bassman AB165/AA864 Tone circuit troubleshooting...
Posted: Wed Mar 16, 2011 9:31 am
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Unsolder the grounded end of the capacitor and check for DC voltage any DC voltage indicates that the cap is worn


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Post subject: Re: Bassman AB165/AA864 Tone circuit troubleshooting...
Posted: Wed Mar 16, 2011 11:36 am
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For coupling caps, I disconnect the lead of the cap going to the next grid stage. You'll notice that one end is usually hooked to the last stage's anode plate and the other to the next stage's signal grid.

Be careful here: you are going to measure voltages with the amp on, not in Standby mode, no inputs, volume(s) at zero, speaker(s) connected.

Hook the red wire of your DVM to the lifted cap lead. The black wire to the chassis ground. Set meter to read Voltage-DC.

With amp on you should only "see" a few mV. If you are reading close to or above 1VDC... you can consider the cap "leaky." If you read on the other lead of the cap (the one that is still soldered to the circuit). You prolly "see" a few hundred VDC. Maybe, somewhere between 100-300 VDC. Best to check schematic, to find what VDC should be read, at which points of the circuit.

This is why it's best to choose good quality, high VDC rated caps (400-600VDC). They have less chance of breakdown and going bad or leaky. You can see the consequences of a leaky cap. Least consequence is resetting the idle bias level of next stage way outta linear region. Worst consequence is a fried tube, when a few hundred VDC is placed onto a grid designed for mV of DC.


OT: I still don't get why the mod calls for 0.001 mfd coupling cap on the PI, in place of the stock 0.1mfd. Hard to believe you'd get any signal passing through such a setup. Oh well...

Good luck on the project!


Last edited by BMW2002Ti on Wed Mar 16, 2011 11:42 am, edited 1 time in total.

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